1. Field of the Invention
This invention relates to containers for goods of the kind having a rigid impermeable base and a flexible impermeable cover which is put over goods on said base and which is sealed to said base.
2. Description of the Prior Art
With this type of container, by evacuating air from the region between the cover and the base, the cover is held down by air pressure onto any goods on the base, as is described, for example, in British Pat. No. 1,191,921 and U.S. Pat. Nos. 3,000,418 and 2,472,623. The cover thereby serves not only as a protective cover but also to secure goods in position on the base. Such an arrangement therefore provides a convenient means not only for storing goods in conditions where they are protected from the ambient atmosphere but also for transport purposes. The base is conveniently made in the form of a pallet to facilitate handling. The cover may be sealed to the base for example in the manner described in U.S. Pat. No. 3,850,214 by providing an endless tube around the periphery of the cover sheet, which tube is put in a slot or groove in said base and inflated so as to be securely held in the slot or groove.
The vacuum required within the cover for storage purposes and to ensure that the cover is held down firmly need only be quite small. The amount of vacuum, that is the amount of pressure difference below atmospheric pressure, may, for example, be of the order of 0.1 to 2.0 lb/square inch and typically would be less than 0.5 lb/square inch. For use for storage purposes, it is satisfactory to provide such containers with a one-way valve permitting connection of the interior of the container to a vacuum pump or other vacuum source. Simple pumping equipment readily permits of evacuation of sufficient air to achieve the required pressure difference. If necessary, the container may be initially filled with dry air or an inert gas to provide a suitable atmosphere for storage purposes.
When such a container is carried by aircraft however, the ambient pressure in the aircraft, when flying at altitude, is typically at a level substantially below the pressure inside the container. As a result therefore the cover, instead of being firmly held down on the goods on the base of the container, is expanded by the higher pressure within the container than in the surrounding atmosphere in the aircraft. It is known to provide the base of the container with posts to enable containers to be stacked and it is convenient for many purposes to provide panels, for example wire mesh panels around the sides and over the top of the container to prevent unauthorised access to the goods. Such panels serve to restrain the cover from expanding too far when the container is being transported in an aircraft.
In some cases however it would be desirable to retain the cover in a condition where it holds the goods down onto a base even during flight in the aircraft. This cannot be achieved merely by the provision of a one-way relief valve permitting air or other gas inside the container to flow out therefrom when the ambient pressure is below that within the container. Such a valve will merely reduce the pressure inside the container to that of the ambient atmosphere; there is no pressure difference and hence the cover no longer serves to hold goods firmly on the base.
In some cases however, a further problem arises. Some goods have to be carried on shock-absorbing mounts, for example resilient mounts, on the base. If the air in the container is evacuated or allowed to escape when the pressure in the aircraft falls substantially below normal atmospheric pressure, then, when the aircraft comes down and the ambient pressure inside the aircraft rises, there is then a very substantial pressure difference between the inside of the container and the surrounding atmosphere. This may cause the cover to be held down so firmly that the shock absorber mounts are compressed to their limits and thereby are no longer effective for protecting the goods against shock loads on the base.
One solution to this problem is disclosed in U.S. Pat. No. 4,117,875 in which suction means are operated continuously and wherein an adjustable bleed orifice open to the atmosphere admits air into a pipe leading from the vacuum suction duct through which air is drawn from the container. The pressure difference between the inside and outside of the container then depends on the pressure drop through the bleed orifice. Such an arrangement however requires the continuous operation of the suction means.